The present invention relates to an electrographic recording process and means, as well as to apparatus for performing the same.
Various electrographic recording papers are at present available on the market (direct electrical recording by means of a stylus forming an electrode).
A first type of paper called sparking papers (such as Teledetos, Western, Union and Timefax papers) consist of a black base covered with white pigments which volatilise or carbonise under the action of an electrical spark caused by the signal to be recorded, and which lead to the appearance of a black mark on the paper. These papers have the advantage of operating dry but have the disadvantage of requiring a high voltage and of giving off unpleasant smells during recording.
A second type of paper consists of electrolytic papers (Mufax, Alden, Hogan) in which the passage of the current causes a chemical reaction leading to the formation of a dye. Various electrolytic mechanisms may be involved, the introduction of ions into the sensitive layer, reactions to the electrodes (cathodic reduction, anodic oxidation) and modification to the pH in the vicinity of the electrodes which may, for example, cause diazo-phenol coupling. Electrolytic papers have the advantage of functioning under a relatively low voltage but in particular have the disadvantage of requiring a high current and a high humidity level (necessity of keeping the paper in hermetically sealed boxes). Furthermore, both these types of papers have the disadvantage of having a limited recording speed due to the time necessary for, or for the transit of ions in the electrolyte of the paper.
A third type consists of dielectric papers functioning on the basis of an electrostatic process similar to Xerox or Electrofax-type photocopying processes. In the case of such papers an electrode raised to a high voltage produces a distribution of local electrostatic charges on an insulating paper. These charges are developed by the passage of the paper through a toner suspension, the latter then being fixed by heat treatment. These papers have the disadvantage of requiring the use of a high voltage, a toner and heat fixing. Moreover, the machines using the same are large, costly and require a high level of maintenance.